1. Field of the Invention
The field of the invention is that of the amplification of ultra-wideband electrical signals from the dc to the microwave range, and notably from the dc range up to microwave frequencies of over 6 GHz.
A preferred application of the invention is in the amplification of signals transmitted by transmission systems at very high bit rates, notably in monomode optic fibers.
The rapid increase in traffic in transmission systems in recent years has fostered the development of optic systems of monomode transmission at very high bit rates. The optic repeaters made for such systems require the installation of fast electronic circuits for modulation at transmission and demodulation at reception. In these repeaters, or optic heads, ultra-wideband amplifiers are needed for the modulation of the transmission laser diode and for the demodulation of the reception photodiode.
The invention can also be used in instrumentation, for example for the making of amplifiers for fast pulses or steps with short rise times, as well as for military applications, notably in the field of video amplifiers, medical applications and, more generally, in all fields where it is necessary to amplify over a very wide band of frequencies.
2. Description of the Prior Art
It is known that the active elements used in the building of dual-access amplifiers (transistors) have the particular characteristic of needing different dc biases at each access.
It therefore becomes necessary, in an amplifier with several stages, to use either high value capacitors, to let through the low frequencies, or dc amplifier topologies to enable the independent biasing of each amplifier stage.
For example, there are known intra-stage capacitor amplifiers such as the amplifiers SHF 74 and SHF 74P, marketed by the firm SHF-DESIGN BERLIN (Trade Name). In such amplifiers, the capacitors used have a value of 10 nF. They can therefore be made only by means of hybrid technology.
The ultra-wide frequency band is obtained by using a feedback loop at each stage, with a 50 pF capacitor in the loop, so that different dc biases can be maintained at each transistor access. Such amplifiers have a flat gain of about 2 dB in a 500 KHz-8 GHz band.
Apart from the problems of their implementation, requiring the use of hybrid circuits, amplifiers with capacitors have to cope with the fact that it is impossible, owing to the very principle of the capacitor, to amplify the low frequencies (between 0 and 500 KHz in this example).
The use of the capacitors may be avoided in dc amplifiers made by monolithic microwave integrated circuit (MMIC) technology, such as those described by Colleran W. T. and A. A. Abidi in "A 3.2 GHz, 26 dB Wideband Monolithic Matched GaAs MESFET Feedback Amplifier Using Cascodes" (IEEE Trans. Microwave Theory Tech., Vol. MTT-36 pp. 1377-1385, October, 1988).
These amplifiers have a series of amplification stages based on gallium arsenide MES field-effect transistors (GaAs MESFETs). The independence of bias between the drain of the active transistor of a stage and the gate of the active transistor of the next stage is achieved by two transistors and a diode bridge. Amplifiers of this type, made by MMIC technology based on GaAs MESFETs with a gate length of 1 .mu.m are limited, in high frequency, to less than 3.5 GHz. The use of a technology based on transistors with a gate length of 0.5 .mu.m extends this upper limit to 5 GHz.
Certain improvements in this technique, enabling substantial increases in efficiency, are known. Thus, for example, it is possible to place a capacitor in parallel on the diode bridge, as is described in the article by Miyagawa et al., "7 GHz Bandwidth Optical Front-End Circuit Using GaAs FET Monolithic IC Technology" (Electronics Letters, Vol. 25, No. 19, Sep. 14, 1989, pp. 1305-1306).
The frequency band of an amplifier such as this can also be increased by about 25%, by using cascode-mounted transistors.
The use of transistors to separate the amplification stages has proved to be very effective with respect to the dc insulation between the stages. By contrast, these transistors go off very swiftly at high frequencies. Furthermore, problems of resonance soon come up. Such amplifiers are therefore limited in the microwave range, owing to their basic principle.
Another way of making an ultra-wideband amplifier is proposed by Kahlert J, Piscalar W. and Mulombe N. in "DC-12.3 GHz Broadband Amplifier" (El. Lett. Vol. 25, No. 21, pp. 1453-1465, Oct. 12, 1989). This is an amplifier with spectral duplexing, in which the lower and upper frequencies are separated by a network made up of capacitors and resistors. That part of the signal that is formed by high frequencies is amplified normally, and the low-frequency component is amplified by a series of operational amplifiers before being re-injected into the gate of the amplification transistor. The network of operational amplifiers is also used to bias this transistor. A two-stage amplifier thus made in hybrid circuit form can be used to obtain a gain of 10 dB.+-.from dc to 12.3 GHz.
The amplifiers with spectral duplexing can be made only by means of hybrid technology. Besides, their design is relatively complex, and they include a relatively large number of basic components.
The invention is designed to overcome these different drawbacks of the prior art.